1. Field of the Invention
This invention relates to a method for the inhibition of popcorn polymer growth by treating a popcorn polymer with a popcorn polymer growth inhibitor while the operation of the apparatus is suspended and the apparatus is maintained in a state destitute of olefins for the purpose of precluding the cumulative adhesion of olefins (referring collectively to all hydrocarbons possessing at least one double bond and derivative thereof) to the recovery system or the refining system.
2. Description of the Prior Art
In the production of an olefin, a so-called popcorn polymer of a porous three-dimensional structure occurs accidentally and not infrequently in the apparatus owing to the polymerization of the olefin in the step of refining and recovery or in the step of recovering the unaltered monomer after termination of the polymerization during the production of synthetic rubber. This popcorn polymer occurs both in the gaseous phase and the liquid phase. It is more likely to occur where the concentration of the monomer (olefin) is high and the temperature is high. A minute amount of oxygen (peroxide) acts as an initiator for the reaction of polymerization and starts the reaction. Iron rust, if any, accelerates the reaction of popcorn polymerization to a great extent.
Numerous monomers containing such olefins as styrene, .alpha.-methyl styrene, acrylic acid and esters thereof, vinyl acetate, acrylonitrile, acrylamide, methacrylamide, etc. and such diolefins as 1,3-butadiene, isoprene, and chloroprene, on reaching refining devices during the step of production and the step of recovery, assume certain conditions such as those related to temperature, monomer concentration, coexistence of vapor and liquid phase, humidity, trace oxygen and iron rust which are highly conducive to the occurrence of popcorn polymerization.
The popcorn polymer is disposed to forming "seeds" which may continue to propagate until the monomer ceases to exist in must the same way as cancerous cells. Because of this nature, minute particles of the popcorn polymer so formed at all rapidly grow into large lumps of polymer. The popcorn polymer therefor adheres to and defiles the heat-exchanger, distillation tower, and piping installed within the system for refining and recovering the produced olefin and deteriorates the efficiency of the refining operation. It often clogs the apparatus and its piping. In an extreme case, the mechanical pressure generated during the propagation of the polymer may deform and fracture the apparatus.
The reason for the rapid propagation of the popcorn polymer is that, as the polymer grows, radically active sites are newly formed inside the polymer and the polymer attains growth from the newly formed radically active sites. It is a surprising fact that the (radically) active sites existing inside the polymer have a long life. When the polymer exposed to the ambient air during suspension of the operation of the apparatus is brought into contact with the monomer as a result of the resumption of the operation, it again starts growing and propagating from the active sites present therein.
The popcorn polymer is of such a quality that it is insoluble in all solvents and defies fusion by heating. For the removal of the mischievous popcorn polymer, the apparatus must be disassembled and mechanically cleaned. Temporary suspension of the apparatus and the cleaning thereof bring about an immense economic loss.
Numerous inhibitors have been proposed for the purpose of precluding the occurrence of this popcorn polymer phenomenon. Examples are nitrites, nitrogen oxides, nitroso compounds, alkyl phenols, aromatic amines, hydroxylamines, etc. For these inhibitors to be effectively used, they must be continuously injected into the apparatus during its operation. These conventional inhibitors curb the growth of the popcorn polymer when the apparatus is clean. They have a serious drawback, however, in that they are deprived of effectiveness in the presence of "seeds" of popcorn polymer. If the popcorn polymer defies being removed by the mechanical cleaning performed during the suspension of the operation, it gives rise to "seeds" for further propagation, and the conventional inhibitors are no longer capable of curbing the propagation of popcorn polymer which starts after the resumption of the operation.